In a vehicle headlight device, it is required to form a predetermined light distribution pattern specified by road traffic rules or the like. “Light distribution” refers to a luminous intensity distribution of a light source with respect to space. That is, it refers to a spatial distribution of light emitted from a light source. “Luminous intensity” indicates the degree of intensity of light emitted by a luminous body and is obtained by dividing the luminous flux passing through a small solid angle in a given direction by the small solid angle. An example of the predetermined light distribution pattern is a light distribution pattern of a low beam. “Low beam” refers to a downward beam, is used in passing an oncoming vehicle or the like, and is also referred to as a headlight for passing each other. Typically, the low beam illuminates about 40 m ahead. To prevent dazzling of an oncoming vehicle, it is required that a cutoff line is formed in the light distribution pattern of the low beam. “Cutoff line” refers to a light/dark separating line or boundary line at an upper end of the light distribution pattern. Specifically, it refers to a light/dark separating line formed at an upper end portion of the irradiated region when a wall or screen is irradiated with light from a vehicle headlight device. Cutoff line is a term used when a radiating direction of the low beam is adjusted. To illuminate an area in a traveling direction of a vehicle particularly brightly, it is also required that the light distribution pattern of the low beam is brightest near and below the cutoff line.
Patent Reference 1 discloses an automobile headlight that reflects light from a light source bulb by a reflecting mirror to radiate it forward and blocks the light by a light blocking plate to form a cutoff line.